I'm curious -- what exactly did your attack victim tell you? I've thought about this situation a lot, since I live in the Pacific Northwest and hike and bike in the deep woods every chance I get. I'd be very interested to hear what they did, and if they were able to use a knife.
I think a knife is a pretty minimal defense against a mountain lion, almost useless on coyotes, and worthless against a snake. Coyotes are wary, and would most likely only attack in a large force, if at all. Snakes (unless you're wrapped in a death roll with a python) aren't good targets for a hack or slash -- better to grab some big rocks and throw them from a distance. That is, if you can't get around the snake. I pretty much feel like if I'm out in the wilds, I'm in their home and if I can avoid them, I will. Around my house -- that's a different matter. As for lions -- unless you have the knife in hand when attacked, and are VERY steady whilst being mauled/shaken, it's going to be hard to use a knife to deter a lion. Better than nothing, though.
I carry a RAT7 D2 inside the outer pouch of my Camelbak M.U.L.E., protruding slightly through the partly opened zipper. I took the leg cord off the sheath and tied it through the handle to secure the knife while chopping, and I leave this hanging outside my pack. I leave the knife unsnapped, so all I have to do is swing the pack around, grab the cord and yank it, and the knife is in my hand. This gives me a large, very sturdy, well made and effective knife to use very quickly. Of more use is the Glock 20 10mm w/15 rounds zipped in behind it. The Glock is by far my preference in a violent encounter, but it's harder to get to. I also carry a small canister of pepper spray in an elastic holster strapped to the front of my Camelbak, for use against aggressive dogs or whatever.
My thought is that you need layers of defense, so you have options very readily at hand for whatever comes up. If you live in a place where concealed weapons permits are harder to come by, you'll have to decide if the risk of carrying a gun to defend yourself, especially in places where you know people have been attacked, is worth the penalty if you have a run-in with the law and get searched.
As for D2 being brittle - I've had no problems using my RAT7 to chop all sorts of stuff. Doesn't mean it's bullet proof - just that I've used it hard and been delighted with it.